The present invention relates to an electrochemical gas sensor having electrolyte and at least two electrodes, in which there is a hydrophobic gas communication means between the electrodes, extending to the electrolyte reservoir of the electrochemical sensor, and especially extending between the electrode and the ambient atmosphere. In particular, the electrochemical sensor uses an air/platinum electrode and an acidic electrolyte, especially sulphuric acid.
Most amperometric electrochemical sensors employ an air/platinum electrode as the reference electrode. Thus, the output from the sensor depends significantly on the potential of this reference electrode. If the reference electrode maintains a constant potential, the sensor will give a stable output that is linear with the concentration of the gas that is to be detected e.g. carbon monoxide.
The potential of the reference electrode is determined by the concentration of hydrogen ions and the partial pressure of oxygen. Any change in the concentration or partial pressure will cause a drift in the output of the sensor in the presence of, for example, carbon monoxide if carbon monoxide is the target gas that is to be detected. If, for example, oxygen is depleted around the reference electrode, the output of the sensor would drop to zero even though the gas (carbon monoxide) concentration remains unchanged. In a three-electrode sensor, the oxygen is reduced to water at the counter electrode. In a two-electrode sensor, oxygen is reduced at the counter electrode, which also serves as a reference electrode.
Buildup of pressure inside a sensor is another problem. Many sensors use an acidic electrolyte combined with a reservoir that is partially filled with electrolyte. Consequently, there is a substantial amount of air in the reservoir, which is separated from the ambient atmosphere by the electrolyte. This liquid-trapped air expands when subjected to elevated temperature, or becomes compressed when the volume of electrolyte increases due to the absorption of water from a humid atmosphere. As it is very difficult for air to dissolve in an aqueous electrolyte, the air pressure inside the sensor increases. A large pressure difference across the sensing electrode can cause leakage of electrolyte either through the sensing electrodes membrane, or through the joints between the parts of the housing in which the sensor is located.
U.S. Pat. No. 4,587,003 discloses the use of both hydrophobic and hydrophobic paths in a sensor for detection of, for example, carbon monoxide, to overcome problems associated with interfering gases contacting the sensing electrode by permitting the interfering gas to also contact the counter electrode. The patent discloses the use of several capillary holes each of a diameter of about 2 mm filled with hydrophobic material in a hydrophilic matrix between the electrodes, and the use of Teflon(trademark)-impregnated fiber glass pads.
In some other sensors, the dosage of electrolyte is strictly controlled so that the adsorbent matrix is not completely saturated with electrolyte and oxygen is supplied to the reference/counter electrode(s) through these dry regions.
Sensors made by these methods have the drawback of difficulty in matrix preparation and unreliable sensor performance. For example, a sensor using a partially dry matrix absorbs water when subjected to high humidity. Consequently, the dry areas can become wet and the gas channel eliminated, with the consequence that the output of the sensor will decrease over a period of time in a target gas e.g. carbon monoxide, due to decreasing partial pressure of oxygen.
To facilitate equalization of pressures inside and outside a sensor, and provide an oxygen path to the electrolyte, U.S. Pat. No. 5,284,566 and U.S. Pat. No. 5,338,429 disclose the use of a small hole in the bottom of a sensor body which is covered by a piece of gas porous but liquid impervious material. Such a design has serious disadvantages. In addition to possible leakage through this hole and increased manufacturing cost, the gas pathway can easily be flooded by electrolyte even when the sensor is sitting in an upright position. Moreover, it provides a passage for a target gas e.g. carbon monoxide, to access the reference electrode by diffusion, causing a drift in the reference electrode potential and a decrease in sensor output. If a poisoning gas enters the sensor, the life and performance of the sensor will be affected.
Alternate methods of ensuring adequate supply of oxygen and for prevention of pressure buildup within a sensor are required.
An electrochemical gas sensor in which hydrophobic means are provided for transmission of oxygen within the sensor and for relief of pressure has now been found.
Accordingly, one aspect of the present invention provides an electrochemical gas sensor having electrolyte, at least two electrodes and an electrolyte reservoir, said electrolyte reservoir containing electrolyte, said electrodes and electrolyte being contained in a housing, said sensor having a hydrophobic gas communication means between said electrodes, electrolyte reservoir and ambient atmosphere.
In an preferred embodiment of the present invention, the hydrophobic gas communication means facilitates equalization of pressure inside the sensor with atmospheric pressure.
In a further embodiment of the invention, the hydrophobic gas communication means provides for transmission of oxygen to electrodes, especially to the reference electrode and counter electrode.
Another aspect of the present invention provides an electrochemical gas sensor having hydrophobic gas communication means, said electrochemical sensor having electrodes and an electrolyte reservoir, said hydrophobic communication means being mounted between the electrodes and extending down to the electrolyte reservoir of the electrochemical sensor.